How old is too old to lead?

The retirement of Pope Benedict XVI, the first such abdication in almost six centuries, may force aging lions from politics, academia and business to confront that painful question. For the legions of leaders well into their 80s and beyond, the evidence isn’t encouraging.

Pope Benedict will step down two months before his 86th birthday because he no longer has the strength to lead the world’s 1.2 billion Catholics, he said Monday.

His decision may prove emblematic of an older generation that is perhaps healthier than earlier generations, yet may harbor unrealistic expectations about what’s physically possible as the body ages.

‘‘The Pope is probably making a wise decision because his ability to totally manage the church is going to be more limited on average because he’s 85,’’ said Leo Cooney, 69, chief of geriatrics at Yale School of Medicine.

Many of the world’s most influential business leaders are confronting the same decision as they continue to rule their empires in their 80s and even 90s.

The scientific community’s consensus on these long-reigning tycoons is clear enough: All good things come to an end. Some lucky ones are vigorous into old age; many others aren’t.

‘‘The prevalence of significant cognitive and functional problems does increase dramatically in the mid-80s,’’ said Cooney. ‘‘It’s probably a good idea to reassess one’s ability to be in positions of power at that time.’’

Life expectancy in the United States is now 76 years for men and 81 for women. Life expectancy is about 78 years in developed countries and 68 years in developing regions, the United Nations said in a report last year.

By 2050, newborns can expect to live to 83 years in developed regions and 74 years in developing regions, it said. Those fortunate to live beyond their ninth decade will face a myriad of difficult hurdles.By the time people reach their 80s, muscle mass, strength and endurance decrease.

Almost 70 per cent of Americans ages 85 to 89 have a disability, defined as a substantial limitation in major life activity, according to a 2011 report on aging by the US Census Bureau and National Institute of Aging.

"‘There’s definitely not a magic number, but there are things that happen more as you age,’’ said Audrey Chun, medical director of Martha Stewart Center for Living at Mount Sinai Medical Center in New York. ‘‘As you start collecting these conditions where you have more than one chronic condition, those things can start taking a higher toll.’’

Eighty-five these days is old, but it’s not terribly old.

As conditions accumulate, so do the medications, which can lead to side effects that can leave people somewhat impaired, Chun said. Dementia and cognitive decline also kick in. By 90 about half of people have some sort of cognitive problem, she said.

Difficulty performing errands alone and mobility-related activities such as walking and climbing stairs are the most common types of disability, affecting two-thirds of people ages 90 and older, according to the Census Bureau and National Institute of Aging study. Hearing and vision loss affect 43 per cent and 26 per cent respectively.

It’s not all bad news, said Thomas Kirkwood, associate dean for aging at Newcastle University in northern England, who has studied aging for almost 40 years.

The university’s Newcastle 85+ Study, which enrolled more than 1000 85-year-olds from the Newcastle and North Tyneside areas, found that on average people had four or five age-related health conditions. Still, about 80 per cent of them rated their health and quality of life as good or excellent, Kirkwood said.

‘‘I do feel some regret that the pope’s decision serves to confirm a negative stereotype we have in society about being old,’’ he said in a telephone interview. ‘‘The common idea that 85 is an age when someone is likely to be past it simply doesn’t stand anymore.’’

The findings suggest that women may face more hurdles then men in the mid- and late 80s. At 85, about 37 per cent of men reported no limitations to their daily living activities, such as cooking, bathing and managing personal finances. The percentage was about half that for women, Kirkwood said.

‘‘It’s a very interesting paradox that although women live longer than men, women at the end of life experience more disability and ill health than men,’’ he said.

Some leadership roles have a tradition of abdication by a certain age. Queen Beatrix of the Netherlands, 75, will step down this year and be succeeded by her eldest son, Crown Prince Willem-Alexander, who will become the country’s first king in more than a century. Beatrix succeeded her mother, who abdicated in 1980.

Most elderly powerful business people, though, have no intention of abdicating if they don’t have to. And that may be the smartest decision they ever make, according to many gerontologists.

Hanging onto a job full of responsibilities and challenges may be the best thing for an executive’s health, said Barbara Messinger-Rapport, director of the Cleveland Clinic’s center for geriatric medicine.

‘‘To age successfully, you have to have cognitive challenges,’’ she said. ‘‘You have to have social engagement, so you’re with peers and have meaningful activities. You need physical challenges to maintain your ability.’’

Indeed, when it comes to aging, no rules apply.

Michael DeBakey, the US cardiovascular surgeon who developed heart-bypass procedures that improved the lives of millions of patients and prolonged life for others, had a heart bypass at 97 and went back to work seven months later. DeBakey died at 99 in 2008.

Former US senator Strom Thurmond retired in 2003 and died later that year at 100. Representative John Dingell, 86, is the longest-serving member of the House.

‘‘We need leadership in the whole business of getting older,’’ said Newcastle University’s Kirkwood. ‘‘Eighty-five these days is old, but it’s not terribly old.’’